Method and installation for taking x-ray exposures



May 9, 1939. 'A. BOUWERS 7 2,157,474

METHOD AND INSTALLATION FOR TAKING X-RAY EXPOSURES Filed March 15, 1935 2 Sheets-Sheet 1 a 36" 255 231 "as 32X isa 52 I/VVf/VTOk Ammr aauwm ATTORNEY.

May 9, 1939. A. BOUWERS METHOD AND msnmuxon FOR TAKING X-RAY EXPOSURES Filed March 15, 1935 2 Sheets-Sheet 2 AME/U MUM/[R5 ATTORNEY.

Patented May 9, 1939 UNITED STATES PATENT QFFICE METHOD AND INSTALLATION FOR TAKING X-RAY EXPOSURES Albert Bouwers, Eindhoven, Netherlands, assign-' or to N. V. Philips Gloeilampenfabrieken, Eindhoven, Netherlands Application March 15, 1935, Serial No. 11,355

- in Germany April 6, 1934 3' Qlaims.

The present invention relates to improved methods and installations for taking X-ray exposures, and more particularly for taking X-ray exposures of high intensity and short duration.

As is well known, X-ray exposures of very short duration require high current intensities, and this-if the operating energy of the X-ray tube is supplied directly from the secondary of a high-voltage transformer, the primary of which 1 is connected to the alternating current-causes very high current surges in the mains during the exposure. Furthermore, such an installation requires very expensive, large capacity, and thus large size transformers not only because of the 15 required large instantaneous operating current,

but also to avoid a large internal voltage drop in the transformer, which would materially reduce the terminal voltage across the X-ray tube.

To avoid such objections, installations are used an in which the large instantaneous current required for the operation ofthe X-ray tube is obtained from a charged condenser. Thereby the condenser can be charged through a rectifier from a high-voltage transformer of small capacity and with a small charging current in a comparatively long charging period; extending over a large number of cycles of the alternating current, whereas the energy stored in the condenser can be discharged through the X-ray tube almost 30 instantaneously and with a large current intensity, in accordance with the requirements of such exposures.

In such condenser installations as are now used, the supply source for charging the con-' 5 denser is disconnected when the charging of the condenser is completed. This because if the high tension source would remain connected during the condenser discharge through the X-ray tube, a practical short-circuiting of the transformer through the rectifying tube and X-ray tube would take place due to the desired lowresistance' of these tubes. This resulting short-circuit current would strongly overload the transformer as well as the mains, and at the same time the 45 voltage across the X-ray tube wouldafter the condenser becomes dischargeddrop to a value which is insufiicient to obtain useful radiation. On the other hand, if the rectifying tubes are given a high internal resistance, practically the an entire transformer terminal voltage would be consumed as the internal voltage drop in the rectifier tubes.

To avoil these drawbacks it has already been suggested to disconnect the transformer before u the discharge of the condenser is started by means of an overload relay actuated by the transformer current, or by making the means for switching on the discharge current dependent on the means for disconnecting the transformer.

Another drawback of present-day condenser 5 installations is that the adjustment of the power supply to the X-ray tube, and thus of the X-ray energy used for the exposure, can only be ob-, tained by varying the charging voltage of the condenser. However the varying of the voltage not only affects the amount of energy but also the hardness of the X-rays generated by the tube, which is objectionable in many cases. Attempts to adjust or dose the X-ray energies used for the exposures, by using ray filters, have not been fully successful as a change in the hardness of the X-rays used in the exposures is not avoided thereby.

Again, if to vary the energy supplied to the X-ray tube, the condenser instead of being fully discharged is permitted to discharge only to a selected lower voltage value, part of the charging energy is wasted; furthermore, to meet the widely varying requirements for the entire field of diagnostic practice, very large capacity condensers would have to be used.

The present invention has for its object an improved condenser installation, whereby not only the above drawbacks are avoided, but important novel results are obtained. 3

In the installation according to the invention, the energy used for the exposure can be varied without varying the voltage across the tube, and thus without afiecting the hardness of the X- rays. 85

Furthermore, the proper dose, i. e., the required exposure energy, is obtained with a fixed capacity condenser of comparatively small size.

According to the invention, I provide an X-ray installation in which the energy passing through 40 the X-ray tube during the exposure is obtained partly from the discharge of a pre-charged condenser and partly from a continuous source of current supply. For example, according to a preferred embodiment of the invention, I provide as a continuous current supply source a high-voltage transformer whose secondary is connected in series with a rectifying device and a condenser, to charge the condenser.- This supply source remains connected to the condenser and 60 to the X-ray tube during thedischarge of the condenser, and to avoid excessive loading of the transformer during the discharge, I provide means to gradually increase the internal resistance of the X-ray tube during the discharge.

This latter means may, for example, cause a reduction of the temperature and thus of the electron emission of the cathode of the X-ray tube, or may comprise an auxiliary electrode provided in the X-ray tube, and an arrangement to vary the potential of the auxiliary electrode to correspondingly vary the internal resistance, and thus the anode current of the tube.

In such a novel installation the current through the transformer during'the discharge of the condenser remains comparatively small, whereas the secondary terminal voltage of the transformer;

and consequently the voltage applied across the X-ray tube, remains at the desiredvalue during the entire exposure. While in such an arrangement the current intensity does not remain constant, this is not a. drawback but an advantage, as the current during the exposure decreases in a most desirable manner, namely, the target surface quickly heats up because of the large initial current, whereas the gradual reduction of the current prevents the overheating of the target and maintains it at substantially its maximum permissible temperature. Thus, in an apparatus according to the present invention the power supplied to the tube has the desirable characteristics disclosed in the copending application of Bouwers and Boldingh Ser. No. 653,538, filed January 25, 1933, now United States Patent No. 2,054,493, dated September 15, 1936.

Furthermore, the apparatus meets the varying requirements, of radiography, whereby while using a capacity of fixed value, for time exposures the energy used in the exposure can be increased above the discharge energy of the condenser by increasing .the exposure time, as in this case the exposure time does not affect the sharpness of the photography.

My invention will be more fully explained with reference to specific examples thereof, illustrated in the accompanying drawings, forming part of the specification, and in which:

Figure 1 is a schematic circuit diagram of an installation according to the invention.

Figure 2 is a schematic circuit diagram of another arrangement in accordance with the invention.

In the drawings similar parts are denoted by similar reference numerals.

The installation shown in Fig. 1 is adapted both for fluoroscopic work as well as for radiographic work by using the same tube and the same source of supply, whereby for fluoroscopic work a small power is supplied'to the tube for a comparatively long period whereas for radiographic exposure, a higher power of short duration is supplied thereto.

Referring now to Fig. 1, the installation shown therein comprises an X-ray tube 4 having an incandescent cathode 9 and an anode l3. The X-ray tube 4 is connected across conductors and 6!, the cathode being connected directly to the conductor 6|, whereas the anode is connected to conductor 50 through a relay switch I actuated by a coil 15. As long as the coil l5 is deenergized the switch I4 is closed, and is opened upon energization of the coil. The switch I4 is a high-voltage circuit switch, adapted to interrupt high-voltage currents preferably without sparking such as a vacuum-type switch.

Also connected across the conductors 60 and BI is a series arrangement of two condensers 'I--'! and of two choke-coils 88. The condensers '|1 represent the condenser or condenser battery for storing the ene gy used for the discharge energy through the X-ray tube, and may represent one condenser or a plurality of condensers of suflicient capacity and adapted to stand the voltage necessary to provide the desired discharge energy through the tube. It should be understood that when referring hereinafter to a condenser", any suitable parallel and/or series combination of condensers is also included.

The choke coils 8-8 are provided to give the discharge current through the X-ray tube the desired characteristic, and the provision of such choke coils is of great importance, as has been explained in the application of Bouwers and Kuntke Ser. No. 551,516, filed July 17, 1931.

Also connected between the conductors 60 and 6| is a bridge arrangement of four high-voltage rectiflers 6, which are connected in known man; ner at opposite corners 62 and G3 to the secondary winding 5" of a high-tension transformer 5 having a primary winding 5' the remaining corners of the bridge being connected to the conductors 60 and El respectively. It should be well understood that any other suitable rectifier arrangement may be used. The rectifiers 6 may be of any suitable type, but are preferably incandescible cathode rectifier tubes having a gaseous, for instance, metal vapor, filling preferably mercury vapour.

The primary winding 5' of transformer 5 is energized from the conductors l6 and I1, connected through switch 90 to the ordinary alternating current mains; one end of the winding 5' being connected through a hand switch l8 to the conductor I6 at 95, whereas the other end of winding 5' is connected to conductor l1 through a conductor 54 and instrumentalities more fully described hereinafter.

The heating current for the cathode 9-01 the X-ray tube 4 is supplied by a heating transformer ID, the secondary winding IQ" of which is connected across the cathode 9. The primary winding I of transformer I0 is connected with one end to the supply conductor l6, whereas its other end is connected through an adjustable resistor II, and through a resistor I! to the supply conductor l'l.

For the convenient operation and regulation of the installation there are provided two switching devices I and 2, each being adapted to occupy two positions.

Switch I when in position I places the apparatus in the condition required for fiuoroscopy, whereas in position II it pro-conditions the apparatusfor the taking of a radiographic exposure.

Switch 2 is inoperative in position I of switch I, whereas in the position II of switch I, it con ditions the installation for high intensity radiographic exposure of a predetermined duration, the duration of which depends upon adjustment of a time switch- 3, later to be referred to.

' The switching device I is provided with three contact'arms 20, 28 and 22 hinged at heels 53, and 56 respectively and insulated from each other.

Heel 50 is connected to supply conductor l1; heel 53 is connected to lead 21 of coil 15, the other lead 26 of which is connected to supply conductor i 8; and heel 56 is connected tothe junction point SI of resistors II and I2.

In position I of the switch i, as shown in the drawings, only the arm 20 is active and contacts with contact 5|, which contact is connected to a contact 69 of a spring-biased, normally open manually-operated switch I9.

A short-circuiting branch is provided across conductors 60 and ti and thus also across the condensers 1--'i, this short-circuiting branch including a switch 34 and a resistor 36. The switch 34 is controlled by a coil 35 and is opened when the coil is energized and closed when thecoil is deenergized.

To operate the .tube for fluoroscopic work, switch 90 is closed, thereby causing energization of heating transformer it through a high resistance circuit, including the resistor II and resistor 82. Thus a moderate electron emission of cathode 9 results, whereby the internal resistance of the X-ray tube remains high and when the high tension is switched on only a small anode current can pass through same.

With switch i in its left-hand, or fluoroscopic, position I, as shown in the drawings, the energizing circuit of coil i is interrupted by open switch arm 25, whereby switch it assumes its closed position. Furthermore, as long as switch it remains in its open position the energizing circuit of coil 35 is interrupted. The energizing circuit of coil 35 may be traced from conductor it, through conductor 96, coil 35,- conductor 91, contact 59, switch i 9, contact 59, contact 5i, switch arm 20, heel Bil to the supply conductor i1. With coil 35 deenergized, switch 34 is closed and condensers i are shunted by the circuit including inductance 8, conductor 60, resistance 86, switch 34, conductor 8i and inductance 8. Furthermore, while switch is is open the energizing circuit of primary winding 6' is interrupted thereby and thus no charging oi condensers i can take place. Thus with switch i in position I, the condensers i are in a discharged state as long as switch i9 remains open.

Hand switch it is normally closed, and upon closure of manual switch it, the primary winding 5' of transformer 5 becomes energized from supply conductor l6 and it through a circuit starting from point 95 of conductor l6 and including switch is, winding 5', leads 64 and 44, switch i9. contactli, switch arm 20, and conductor li. Closure'of switch It also energizes coil 35 by closing its energizing circuit through switch i9 and switch arm 20 which is in contact with contact 5i. Coil 35 being energized, switch 34 is open and the shunt circuit of condensers I through resistance 36 is interrupted. v

In this condition a rectified voltage, corresponding to the full secondary voltage or the transformer. is appliedto the X-ray tube, but because of the high internal resistance of the tube, only a small current passes through same. Thus a comparatively small power, as is required for radioscopy, is supplied to the X-ray tube as long as switch it remains closed.

While during the closure of switch it the con-' densers are being gradually charged, they do not function during radioscopy. Y

when the switch I is moved into its position marked II (which is the position for radiography), the arms 20, 28 and 22 contact with contact points 52, 55, and El respectively. Contact 52 is connected to a time switch I, and is also connected to lead 64 of primary winding 8' and thrdugh lead 44 to coil 45.

The switch 2 is biased in its lower-position, as shown in the drawings by spring ll and is raised into its upper position upon energization of coil ii. The switch 2 carries two contactors H and 12, which are insulated from each other. When coil II is deenersized contactor 12 connects contact 2! with contact 84, the latter'being connected sistor 62.

The energization of coil 3| takes place through an adjustable timing device 8, connected in series with the coil 3|; one end of this series connection being permanently connected to supply conductor it at '95, wheras its other end is connected to supply conductor i'ionly when switch i is in its position II-through contact 52 and arm 20 of switch I. The coil 3| is energized for a limited time interval, depending upon the adjustment and actuation of the time switch 3. Time switches as used in such applications are well known, and

' detailed description of time switch 3 is therefore deemed unnecessary.

It should be noted that the time switch 3 can only actuate coil ti if switch I assumes its position II.

To make radiographic exposures requiring a.

high intensity, short-duration energization of the X-ray tube, the switch i is brought into its position II. This brings about an increase of the cathode emission of the X-ray tube by shortcircuiting the entire resistance i2; this shortcircuiting being established through a branch circuit going, from the end at of resistor i2 through contact arm 22, contact 58, lead 92, con-- tact strip 23, contactor ii, contact 25, and through lead 25 to supply conductor i'i. The result is that the internal resistance of the X-ray tube is reduced to a small value and upon closure of the anode circuit of the X-ray tube a large operating current may pass through same.

For the time being, however the anode circuit of tube 4 remains interrupted as coil I5 is energized through connection of lead 27, contact arm 28, contact 55, lead 93, contact 29, contactor 12, contact 30 and lead 25 to supply conductor i1.

Thus, with switch l4 open, notwithstandingthat the cathode of tube 4 is highly emitting,

no anode current can pass through the X-ray tube.

In the meantime, however, the condensers 1-1 are being charged up as the secondary winding 5" 01' transformer 5 is energized, and switch 34 is open and interrupts the shunt circuit of condensers I. The energization of transformer 5 and of coil ii-to open switch 34takes place by contact arm 20 interconnecting contacts and 52; switch i being in position 11. In this case the energizing circuit of primary winding 5' maybe traced from point 95 of supply conductor it, through hand switch i8, winding 5', conductor 64 to point 84, contact point 94', contact 52, throughcontact arm 20 to the conductor II. The energizing circuit of coil 35 may betraced from supply conductor l6, through conductor 96, coil 35, conductor 91, conductor-44, points 94 and 94' and through contact arm 20 to conductor l1.

With the cathode 9 of the X-ray tube brought into a state of high incandescence, the tube resistance is reduced to a low value, and after the condensers have been charged in a plurality of cycles of the alternating current, the radiographic exposure can be initiated upon actuation of time svgtgh 3.

e time switch 3 is set in known manner to the proper exposure time, which, in accordance with the invention, usually exceeds the duration of the condenser discharge. During the time, set by the time switch3, the coil 3| is energized due to end 94' of the series combination of coil 3| and time switch 3 being connected through contact 52, arm 20, and contact 50 to supply conductor I1.

Energization of coil 3| raises the switch 2 against the bias of spring 10, thereby breaking the connections between contacts' 29 and 30, and contact strip 23 and contact 24 respectively, and establishing connection between contact strip 23 and contact 32.

Interruption at contacts 29 and 30, causes deenergization of coil l5, which closes switch I4,

thereby establishing the anode circuit, and permitting the condenser to discharge through the X-ray tube.

The choke coils 8-8 thereby serve to give the discharge the desired character in a manner described in the above referred to application Ser. No. 551,516. The discharge energy passing through the X-ray tube may be very high, for example, the average energy during the first 0.1 second of the discharge may be 30 kilowatts.

Simultaneously, in accordance with the invention, the internal resistance of the X-ray tube 4 is increased due to the strip 23 being now connected to contact 32 instead of to contact 24: thus the resistor l2, which has been completely short-circuited in the prior condition of the apparatus, is now partly included in the primary cirouit oi the heatingtransforrner in. This reduces the heating current through the cathode 9, in accordance with the value of the resistance portion which falls between the adjustable contact I3 and that end of resistor l2 which is connected to supply conductor l1.

By thus increasing the internal resistance of the X-ray tube and thus the resistance of its anode circuit, the rectified current supplied directly from the secondary winding 5", will have sufiiciently small values to prevent a voltage drop in the transformer'which would substantially reduce the voltage across conductors 60 and 6|. Thus the terminal voltage across the X-ray tube will remain substantially unchanged during and after the discharge of the condenser as long as transformer 5 supplies current to the tube.

Thus in this arrangement, according to the invention the initial current passing through the X-ray tube is due mainly to the discharge'energy of the condenser. This current passes almost instantaneously upon closure of the switch H in a time period determined by the constants of the discharge circuit. The rectified current supplied from the secondary winding 5" is superimposed on the discharge current, and as the discharge progresses the rectified current forms an increasingly large portion of the energy passing through the X-ray tube. While the voltage across the X-ray tube remains substantially unchanged, the current, because of the increasing internal resistance of the tube, gradually decreases.

The proper adjustment of the decrease of the heating current, can be obtained by the proper adjustment of tap I3 of resistor l2. If desired the entire resistor 12 can be again inserted in the circuit, or the heating circuit may be altogether interrupted.

1s The reduction or the heating current, or its interruption, causes the cathode temperature to decrease rapidly, i. e., in a time period which is short with respect to the exposure time of shorttime exposures, but gradually, i. e., a finite time is required for the cathode to assume its lower temperature, although the interruption or reduction of the heating current is substantially instantaneous. This results in a decrease in the electron emission, and a corresponding increase in the tube resistance; whereby by proper selection of the tube and cathode design, and primarily by the selection of the thermal capacity of the cathode body, the rate of the cathode temperature drop and of the increase of the tube resistance can be determined in accordance with the requirements. Thus, for example, after the condenser discharge has taken place a stationary condition is obtained during which the load of the transformer may amount to about 5 kilowatts.

After the desired period of exposure, which has been determined by the proper setting of the time switch 3 and which may be a fraction of a second or a few seconds, has elapsed, coil 3i is deenergized and the switch 2 is returned by spring 10 to its original lower position, thereby re-energizing coil 15 by closing its circuit at 29-30. As a result of this the switch I4 opens and interrupts the anode circuit of the X-ray tube. The switch 35 and transformer 5 remain energized and thus the condensers are again charged and with a new setting of the time switch a second exposure can be made, or by returning the switch I into its position I the installation is ready for fluoroscopic work; whereby closure of switch 34 establishes a short circuit across the X-ray tube as long as switch 19 is open.

While in the arrangement described the primary circuit of the high-voltage transformer remains connected to l6l1, as long as switch I is in position 11 it is also possible to interrupt this circuit after the desired exposure period, for example, by providing in this circuit contacts controlled by the time switch 3. Such arrangement being easily understood does not require illustration.

If it is desired to take a radiographic exposure purely by condenser discharge, without supplementing it by a rectified current supplied from the high-voltage transformer, the normally closed switch I 8 is opened after the charging of the condenser has been completed, but prior to the actuation of the time switch.

If it is desirable to obtain extremely short duration exposures the condenser can be shortcircuited after partial discharge by deenergizing coil 35 after partial discharge, for example by providing in the circuit of coil 35 contacts which are controlled by time switch 3.

Figure 2 shows an arrangement which is similar to that of Figure 1 except that conductor 64 is not connected directly to the conductor 44, but to a conductor 91 leading to the conductor 44 and to the contact 59. Other differences between Figs. 1 and 2 'will appear during the following description. In this installation the gradual increase of the internal resistance of the X-ray tube during radiographic exposure is brought about by an auxiliary electrode or grid 31 provided in the tube, and which may be, for example, a focussing device partially surrounding the cathode. In such a tube if the auxiliary electrode is given a certain negative potential the X-ray tube will assume a high internal resistance, whereas if the grid potential becomes less negative or positive with regard to the cathode, the

resistance of the X-ray tube correspondingly decreases.

The potential applied to auxiliary electrode 31 may be supplied by means of an auxiliary transformer 38 whose primary is connected across the supply conductors l6 and I]; the secondary winding of transformer 38 being connected into a circuit comprising in series a rectifier 39, a condenser 40, and preferably also a resistance 4!.

One side of the condenser is connected at 82 to the cathode 9 of the X-ray tube, whereas its other side is connected through conductor 83 to the auxiliary electrode of the tube. Thus the condenser 40 interconnects the cathode and the auxiliary electrode of the X-ray tube.

In this arrangement the secondary winding 5" of transformer 5 is shown to be connected in series with a charging circuit comprising the rectifier tube 6, condenser 1, and choke coil ii; the condenser and choke coil being connected at and GI across the X-ray tube in a manner similar to that explained in connection with Figure 1. If desired aglow discharge lamp s2 is also connected across the condenser 40 for reasons more fully explained hereafter.

In position I of switch I the condenser 60 is charged and the auxiliary electrode 3? is given a negative potential, thereby providing for a high internal resistance of the X-ray tube.

In position II of switch 6, the condenser 40 is short-circuited by placing across itby means of contact arm 2! a short-circuiting branch circuit including lead 8?, arm 2i, resistor 63, bridged contacts 84 and 85 to lead 83. Due to the shortcircuiting of condenser 40 the auxiliary electrode Si becomes less negative and the internal resistance of the X-ray' tube is decreased to the value desired for the initial high current intensity discharge.

When the time switch 3 is set and actuated in the manner described in connection with Figure 2, the short-circuit branch across condenser 40 is temporarily broken at 84-85, whereby the auxiliary electrode 31 again assumes its previous negative potential. The increased tube resistance limits the current passing through the tube in the manner previously set forth.

When the exposure has been terminated, the

switch I is returned to its position I, interrupting the current flow through coil 35, which provides for a short-circuiting of the X-ray tube a.

It should be well understood that before the time switch 3 is actuated the condenser 40 should be discharged, so that the internal resistance of the tube has the required low value. The glow discharge lamp 42, connected in parallel with the condenser, serves to indicate this, the lamp 42 extinguishing when the condenser is discharged, and conversely, when the condenser 40 is charged an to a determined value, lamp-42 ignites and indicates that the resistance of the X-ray tube has assumed a sufllclently high value to permit the switch [9 to be closed for fluoroscopic examination.

While in the arrangement of Fig. 2 the rectification of the alternating current takes place by means ota single rectifier tube, the anode 01' which together with the incandescent cathode of the X-ray tube is grounded at 45, the bridge connection shown in Fig. 1 using four rectifiers, may also be used in connection with the arrangement of Fig- 2. Conversely, in Fig. 1, instead of using bridge-connected rectifier-s, a single rectifier or rectifiers in any other arrangements 7 may also be used. 4

While I have described my invention on hand of specific embodiments and in specific applications, I do not wishto be limited thereto, but

of the tube to maintain the voltage applied to the tube at a value sufiicient for generating efiective X-rays during the entire exposure.

2. In the process of taking X-ray exposures, the steps comprise charging a condenser over a plurality of cycles and through a rectifier from a small-capacity, high-voltage transformer, increasing the cathode temperature to impart to the tubeai low internal resistance, discharging said charged condenser through said tube and also passing through said tube a rectified current supplied from said transformer, while decreasing the cathode temperature to maintain the voltage applied to the tube at a value sufiicient for generating efiective X-rays during the entire exposure.

3. In an X-ray installation for fluoroscopy and radiography, an X-ray tube having an anode and an incandescible cathode, an anode circuit for said tube, a source of continuous unidirectional current and a condenser associated with said anode circuit, means for charging the condenser, means for controlling said anode circuit including a two-position switch, said switch in one position causing said source to be connected to said tube and establishing the connections for fluoroscopy and in its second position causing interruption of the anode circuit and establishing the connections for radiography, and means for increasing the internal resistance of the tube and establishing the anode circuit for a predetermined time interval, said latter means including a time switch operative only when said two-position switch is in its second position, and a switching device controlled by said time switch.

4. In an X-ray installation for fiuoroscopy and for radiography, a single X-ray tube having an anode and an incandescible cathode, an anode circuit for said tube, a source of continuous unidirectional current associated with said anode circuit and including a high-voltage transformer having a primary winding, and a source of alternating current connected to said winding, a condenser associated with said anode circuit, means for charging said condenser, means for controlling said anode circuit including a twoposition switch, said switch in one position causing said source to be connected to said tube and establishing the connection for fluoroscopy, and in its second position causing interruption of said circuit and'a reduction in the internal resistance of the X-ray tube and establishing the connections for radiography, and means for establishing said anode circuit for a predetermined time interval to make an X-ray exposure with said X-ray tube, said latter means comprising a time switch and a switching device controlled thereby, said time switch being operative only when said twoposition switch is in its second position.

5. In an X-ray installation for radioscopy and radiography, an X-ray tube having an anode and an incandescible cathode, an anode circuit for said tube, a heating circuit for said tube including a resistance to reduce the heating current of the cathode during fluoroscopy, a source of unidirectional current including a high-voltage transformer having a secondary winding, and a primary winding adapted to be connected to a source of alternating current, a secondary circuit for said transformer including said secondary winding, a condenser and rectifying means in said circuit, switching means including a twoposition change-over device, said switching means in one position of said device connecting said primary winding to the alternating current source and in its second position short-circuiting at least part of said resistance and interrupting the anode circuit of the X-ray tube, a time switch operative only when said switching device is in its second position, and a switching member controlled by said time switch to establish the anode circuit of the tube and remove at least part of the short circuit of said resistance for a predetermined' time. a

6. In the process of making an X-ray exposure with an X-ray tube having a control grid, the

steps of charging a condenser at a slow rate through a rectifier from a small-capacity highvoltage transformer, applying a potential to the grid to impart to the tube a low internal resistance, discharging the charged condenser through the X-ray tube and also passing through the tube a rectified current supplied directly from said transformer while gradually increasing the potential of the grid during the condenser discharge to increase the internal resistance of the tube and thereby maintain the voltage across the tube at a value sufficient for generating effective X- rays during the entire exposure.

7. In an X-ray installation, an X-ray tube, a high-voltage transformer having a primary winding adapted to be connected to a source of alternating current and a secondary winding, a secondary circuit for said transformer including said secondary winding, a rectifier and a con- 

